The embodiments described herein generally relate to the magnetic resonance (MR) field, in particular to a method and device for pre-distorting an exciter as well as to a predistortion exciter.
In a known MR system, the radio frequency (RF) amplifier outputs the RF excitation which generates a B1 field. However, due to the nonlinearity of the RF amplifier, the actual output of the RF amplifier will have a distortion as compared to the input, leading to a poor profile of a slice. That is, in the MR system, the RF amplifier has a more complicated distortion, thus the profile of the slice is different from that in the ideal case.
At present, researchers have provided some methods to solve the above-mentioned problems.
For example, U.S. Pat. No. 6,882,221 discloses an apparatus and a corresponding method for predistorting an input signal applied to a radio frequency (RF) power amplifier to compensate for distortion of the amplifier at high powers. The input and output signals of the RF amplifier are continuously monitored and difference signals are generated in an RF phase discriminator. The difference signals are then converted to digital form and stored in a lookup table in the form of running averages of again compression value of the RF amplifier and an output phase value of the RF amplifier for each observed RF input power value. A predistorter module retrieves these values and predistorts the input of the RF amplifier by way of compensation. This patent focuses on applying the predistortion at carrier frequency rather than at baseband, and does not consider how to apply predistortion at a system level.
The U.S. Pat. No. 7,085,330 discloses a method and apparatus for linearization of an amplifier using adaptive predistortion, wherein a signal processing method and apparatus capable of correcting signal distortion introduced by an RF power amplifier is disclosed. The apparatus includes a buffer to store a plurality of samples representing at least a portion of an input signal to be amplified by the RF power amplifier, a receiver to receive an output signal generated by the RF power amplifier, a synchronization unit to determine, as a matching input sample, which of the stored plurality of samples corresponds most closely to the output signal, and a predistortion unit to selectively apply a distortion correction function to the input signal prior to amplification by the RF power amplifier. The distortion correction function is derived from a relationship between the matching input sample and the output signal. This enables more precise and updateable determination of the delays involved in the RF modulation and amplification stages of the amplifier and the self-receiver. A phase offset correction is optionally provided to correct a phase offset in the realized sample of the output signal relative to the matching input symbol. Additionally, a sampling phase error correction unit may be provided to generate sampling alteration information to an analog-to-digital converter to cause such analog-to-digital converter to selectively alter sampling of the output signal. This patent recites the method and apparatus for use in the wireless communication, but it does not mention using them in the MR field.